As semiconductor devices become more highly integrated, there is a decrease in area and wire width of individual semiconductors. The result is reduced substrate area available for capacitor formation. Conventionally, if the electrode area is decreased capacitance is also decreased.
In a semiconductor memory device using a capacitor, e.g. a DRAM, it is necessary to keep capacitance above a predefined level to increase memory operation performance and to decrease power consumption. To satisfy these conflicting requirements, the capacitor electrode can be formed into a stack, a cylinder or a trench. A trench is a deep formation having a sidewall. Likewise, the capacitor bottom electrode can be formed into a complex dented shape or can be protruded at the surface.
Protrusions at the surface of the bottom electrode are formed with hemispherical grain (HSG) using a crystalline boundary of polycrystalline silicon. In the HSG formation method, amorphous silicon is first deposited to form a capacitor bottom electrode and heat treatment is performed at low pressure. A polysilicon layer having HSG is then formed at the surface by controlling temperature and pressure. This type of HSG formation is achieved by heat treatment and deposition, which cause the migration of silicon atoms and result in decreased surface area of the polycrystalline silicon.
U.S. Pat. No. 5,770,500 discloses a method of forming a germanium-doped amorphous silicon layer instead of a pure amorphous silicon layer during formation of a capacitor bottom electrode. According to that method, germanium atoms, under pressure, allow silicon atoms to move easily into a silicon germanium amorphous layer. Germanium also lowers the active energy required for polycrystallization, and as a result helps the HSG grow faster at the amorphous layer surface.
However, HSG formation is difficult to control. If the HSG is excessively formed electrical shorts can occur between the capacitor bottom electrode and other conductive structures, including neighboring bottom electrodes. Excessive HSG formation can also cause a neck to form at the connection site between an HSG protrusion and a bottom electrode. When this occurs, the HSG protrusion is easily disjoined, generating particles and resulting in process failure. Because of these problems, the HSG formation method is infrequently used for highly integrated semiconductor memory devices.